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DOE OSTI2025

Orbital-Selective Instabilities and Spin Fluctuations at the Verge of Superconductivity in Interlayer-Expanded Iron Selenide

Understanding electron correlation-driven instabilities and their coupling to structural phases is essential for deciphering multiorbital pairing in unconventional superconductors. We investigate Li x (C 5 H 5 N) y Fe 2 Se 2 (x ∼ 0.6; y ∼ 0.7−0.9), a tetragonal β-FeSe intercalate with a superconducting transition temperature (T c = 39 K) closely tied to an expanded Fe-layer spacing (∼11.4 Å). High-resolution synchrotron Xray diffraction and core-level absorption spectroscopy reveal subtle lattice distortions on cooling without a symmetry-breaking transition. Instead, the material exhibits negative thermal expansion (NTE) in the two-dimensional Fe network below T S ∼ 70 K, and stiffening of local Se−Fe−Se bond dynamics near T c . The spatially incoherent rearrangement of FeSe 4 tetrahedra and the site-local fluctuations, signal reduced electron correlations compared to those of parent β-FeSe (T c = 8 K). Complementary X-ray emission spectroscopy, a fast local probe of Fe 3d valence states, detects persistent local Fe spin moments below T S , unlike quenching in related systems. These findings indicate that decoupling of Fe planes leads to an electronically driven lattice instability. The latter emerges as NTE induced from weak, orbital-selective localization of in-plane Fe 3d states rather than conventional transverse vibrations. Governed by Hund’s coupling, this selectivity permits coexistence of local spin fluctuations with itinerant d-electrons critical for enhancing T c . These results suggest that intercalation-driven d-orbital differentiation moderates electron correlations, providing a pathway to optimize the superconductivity in low-dimensional quantum materials.

36 MATERIALS SCIENCE↗

DOE OSTI2020

Electrically controlled two-dimensional electron-hole fluids

We study the electronic properties of dual-gated electron-hole bilayers in which the two layers are separated by a perfectly opaque tunnel barrier. Combining an electrostatic and thermodynamic analysis with mean-field theory estimates of interacting system chemical potentials, we explain the dependence of the electron and hole densities on the two gate voltages. Because chemical potential jumps occur for both electrons and holes at neutrality, there is a finite area in gate voltage parameter space over which electron and hole densities are equal. In that regime the electron-hole pair density depends only on the sum of the two gate voltages. As a result, we are able to explain a recent experimental study of electrically controlled bilayers by allowing for interlayer tunneling and using a nonequilibrium steady-state transport picture.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2024

New insight into tuning magnetic phases of RMn6Sn6 kagome metals

Abstract Predicting magnetic ordering in kagome compounds offers the possibility of harnessing topological or flat-band physical properties through tuning of the magnetism. Here, we examine the magnetic interactions and phases of ErMn 6 Sn 6 which belongs to a family of R Mn 6 Sn 6 , R = Sc, Y, Gd–Lu, compounds with magnetic kagome Mn layers, triangular R layers, and signatures of topological properties. Using results from single-crystal neutron diffraction and mean-field analysis, we find that ErMn 6 Sn 6 sits close to the critical boundary separating the spiral-magnetic and ferrimagnetic ordered states typical for non-magnetic versus magnetic R layers, respectively. Finding interlayer magnetic interactions and easy-plane Mn magnetic anisotropy consistent with other members of the family, we predict the existence of a number of temperature and field dependent collinear, noncollinear, and noncoplanar magnetic phases. We show that thermal fluctuations of the Er magnetic moment, which act to weaken the Mn-Er interlayer magnetic interaction and quench the Er magnetic anisotropy, dictate magnetic phase stability. Our results provide a starting point and outline a multitude of possibilities for studying the behavior of Dirac fermions in R Mn 6 Sn 6 compounds with control of the Mn spin orientation and real-space spin chirality.

36 MATERIALS SCIENCE↗

DOE OSTI2021

Isolating the Electrocatalytic Activity of a Confined NiFe Motif within Zirconium Phosphate

Unique classes of active-site motifs are needed for improved electrocatalysis. Herein, the activity of a new catalyst motif is engineered and isolated for the oxygen evolution reaction (OER) created by nickel–iron transition metal electrocatalysts confined within a layered zirconium phosphate matrix. It is found that with optimal intercalation, confined NiFe catalysts have an order of magnitude improved mass activity compared to more conventional surface-adsorbed systems in 0.1 m KOH. Interestingly, the confined environments within the layered structure also stabilize Fe-rich compositions (90%) with exceptional mass activity compared to known Fe-rich OER catalysts. Through controls and by grafting inert molecules to the outer surface, it is evidenced that the intercalated Ni/Fe species stay within the interlayer during catalysis and serve as the active site. After determining a possible structure (wycherproofite), density functional theory is shown to correlate with the observed experimental compositional trends. It is further demonstrated that the improved activity of this motif is correlated to the Fe and water content/composition within the confined space. This work highlights the catalytic enhancement possibilities available through zirconium phosphate and isolates the activity from the intercalated species versus surface/edge ones, thus opening new avenues to develop and understand catalysts within unique nanoscale chemical environments.

36 MATERIALS SCIENCE↗

DOE OSTI2018

Interplay between orbital-quantization effects and the Fulde-Ferrell-Larkin-Ovchinnikov instability in multiple-band layered superconductors

We explore superconducting instability for a clean two-band layered superconductor with deep and shallow bands in the magnetic field applied perpendicular to the layers. In the shallow band, the quasiclassical approximation is not applicable, and Landau quantization has to be accounted for exactly. The electronic spectrum of this band in the magnetic field is composed of the onedimensional Landau-level minibands. With increasing magnetic field the system experiences series of Lifshitz transitions when the chemical potential enters and exits the minibands. These transitions profoundly in uence the shape of the upper critical field at low temperatures. In addition, the Zeeman spin splitting may cause the nonuniform state with interlayer modulation of the superconducting order parameter (Fulde-Ferrell-Larkin-Ovchinnikov state). Typically, the quantization effects in the shallow band strongly promote the formation of this state. The uniform state remains favorable only in the exceptional resonance cases when the spin-splitting energy exactly matches the Landau-level spacing. Furthermore, for specific relations between electronic spectrum parameters, the alternating FFLO state may realize, in which the order parameter changes sign between the neighboring layers. For all above cases, the reentrant high-field superconducting states may emerge at low temperatures if the shallow band has significant contribution to the Cooper pairing.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2025

Pressure-induced structural phase transitions in CrSBr

There is growing interest in combining chemical complexity with external stimuli like pressure, field, and light for property control in van der Waals solids. This is because extreme conditions trigger the development of new states of matter and functionality. In this work, we bring together synchrotron-based infrared absorption, Raman scattering, and diamond anvil cell techniques with first-principles calculations of the lattice dynamics and energy landscape to reveal the series of structural phase transitions in CrSBr. By tracking how the phonons change under pressure, we uncover a remarkable chain of complex symmetry modifications, interlayer interactions, and chemical reactions. A group-subgroup analysis suggests that CrSBr undergoes an orthorhombic Pmmn → monoclinic P2/m transition at 7.6 GPa, and based upon a comparison with model oxychlorides like FeOCl and CrOCl, we propose that changes in the pendant halide groups drive the system to a P2 1 /m-like space group above 15.3 GPa. Compression above 20.2 GPa is irreversible, resulting in the formation of an entirely new compound that is metastable for months. This work opens the door to the use of pressure and possibly strain to control the properties of CrSBr.

Microscopy↗

NASA NTRS1971

The fabrication, testing and delivery of boron/epoxy and graphite/epoxy nondestructive test standards

A description is given of the boron/epoxy and graphite/epoxy nondestructive test standards which were fabricated, tested and delivered to the National Aeronautics and Space Administration. Detailed design drawings of the standards are included to show the general structures and the types and location of simulated defects built into the panels. The panels were laminates with plies laid up in the 0 deg, + or - 45 deg, and 90 deg orientations and containing either titanium substrates or interlayered titanium perforated shims. Panel thickness was incrementally stepped from 2.36 mm (0.093 in.) to 12.7 mm (0.500 in.) for the graphite/epoxy standards, and from 2.36 mm (0.093 in.) to 6.35 mm (0.25 in.) for the boron/epoxy standards except for the panels with interlayered shims which were 2.9 mm (0.113 in.) maximum thickness. The panel internal conditions included defect free regions, resin variations, density/porosity variations, cure variations, delaminations/disbonds at substrate bondlines and between layers, inclusions, and interlayered shims. Ultrasonic pulse echo C-scan and low-kilovoltage X-ray techniques were used to evaluate and verify the internal conditions of the panels.

Pless, W. M.↗

DOE OSTI2019

Energy Expenditure Upon the Formation of the Elastically Stressed State in the Layers of a Step-Graded Metamorphic Buffer in a Heterostructure Grown on a (001) GaAs Substrate

On the basis of data on X-ray structural analysis performed by the method of reciprocal-space mapping and investigations using secondary-ion mass spectrometry and transmission electron microscopy, it is shown that vertical compressive stresses also arise in a multilayer epitaxial heterostructure comprised of a step-graded metamorphic buffer along with lateral compressive stresses. The cause of the appearance of vertical stresses is the effect of interlayer hardening, which arises due to the deceleration of fragments of glide dislocations by interphase boundaries. Analysis performed within the framework of the linear theory of elasticity shows that the elastically stressed state of the buffer steps is similar to the state that can be achieved as a result of a two-stage deformation process: bulk and biaxial compression. Bulk compression leads to large energy expenditures in the formation of the structure of the buffer steps, which is reflected, in particular, in violation of the coherence between the dislocation-free and the underlying layers.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2022

Origin of the magnetic couplings for the weak ferromagnet Li + [TCNE] •- (TCNE = Tetracyanoethylene)

Based upon the 16-K crystal structure, the nearest-neighbor spin couplings (J; H = –2JS a •S b ) for the weak ferromagnet (=canted antiferromagnet) Li + [TCNE] •- (TCNE = tetracyanoethylene) (T c = 21 K), which possesses two interpenetrating diamondoid sublattices with two layers of parallel [TCNE] •- s canted with respect to each other by ~60°, are computed at the B3LYP/aug-cc-pV-TZ-level. Li[TCNE] has computed interlattice, through-space interactions that exceed the intralattice, through-bonding to Li + interactions, that are antiferromagnetic. The interlattice interactions dominate the spin coupling leading to the observed weak ferromagnetic behavior. Finally, the computed J's based upon the 50-K structure have the strongest ferromagnetic intralayer interlattice interactions increase by 2.9 ± 0.6% while the antiferromagnetic interlayer interlattice interaction increases by 23% with respect to the 16K results and are in accord with the observed initial increase and subsequent decrease in the temperature dependence of the canting angle that is a consequence of the these changing opposed interactions arising from the change in structure as a function of temperature.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2024

THz generation by exchange-coupled spintronic emitters

Abstract The mechanism of THz generation in ferromagnet/metal (F/M) bilayers has been typically ascribed to the inverse spin Hall effect (ISHE). Here, we fabricated Pt/Fe/Cr/Fe/Pt multilayers containing two back-to-back spintronic THz emitters separated by a thin ( t Cr ≤ 3nm) wedge-shaped Cr spacer. In such an arrangement, magnetization alignment of the two Fe films can be controlled by the interplay between Cr-mediated interlayer exchange coupling (IEC) and an external magnetic field. This in turn results in a strong variation of the THz amplitude A , with A ↑ ↓ reaching up to 14 times A ↑ ↑ (arrows indicate the relative alignment of the magnetization of the two magnetic layers). This observed functionality is ascribed to the interference of THz transients generated by two closely spaced THz emitters. Moreover, the magnetic field dependence A ( H ) shows a strong asymmetry that points to an additional performance modulation of the THz emitter via IEC and multilayer design.

Adam, Roman↗

DOE OSTI2022

Low-energy moiré phonons in twisted bilayer van der Waals heterostructures

We develop a low-energy continuum model for phonons in twisted moiré bilayers, based on a configuration-space approach. In this approach, interatomic force constants are obtained from density functional theory (DFT) calculations of untwisted bilayers with various in-plane shifts. This allows for efficient computation of phonon properties for any small twist angle, while maintaining DFT-level accuracy. Based on this framework, we show how the low-energy phonon modes, including interlayer shearing and layer-breathing modes, vary with the twist angle. As the twist angle decreases, the frequencies of the low-energy modes are reordered and the atomic displacement fields corresponding to phonon eigenmodes break translational symmetry, developing periodicity on the moiré length scale. Here, we demonstrate the capabilities of our model by calculating the phonon properties of three specific structures: bilayer graphene, bilayer molybdenum disulfide (MoS 2 ), and molybdenum diselenide-tungsten diselenide (MoSe 2 -WSe 2 ).

2-dimensional systems↗

DOE OSTI2016

Short- and Long-Range Attractive Forces That Influence the Structure of Montmorillonite Osmotic Hydrates

Clay swelling is a colloidal phenomenon that has a large influence on flow and solute migration in soils and sediments. While models for clay swelling have been proposed over many years, debate remains as to the interaction forces that combine to produce the observed swelling behavior. Using cryogenic transmission electron microscopy (cryo-TEM) and small-angle X-ray scattering, we study the influence of salinity, in combination with layer charge, interlayer cation, and particle size, on montmorillonite swelling. We observe a decrease in swelling with increased layer charge, increased cation charge, and decreased cation hydration, each indicative of the critical influence of Coulombic attraction between the negatively charged layers and interlayer cations. Cryo-TEM images of individual montmorillonite particles also reveal that swelling is dependent upon the number of layers in a particle. Calculations of the van der Waals (vdW) interaction based on new measurements of Hamaker coefficients confirm that long-range vdW interactions extend beyond near-neighbor layer interactions and result in a decrease in layer spacing with a larger number of layers. This work clarifies the short- and long-range attractive interactions that govern clay structure and ultimately the stability and permeability of hydrated clays in the environment.

58 GEOSCIENCES↗

NASA NTRS1993

Degradation of optical components in space

This report concerns two types of optical components: multilayer filters and mirrors, and self-scanned imaging arrays using charge coupled device (CCD) readouts. For the filters and mirrors, contamination produces a strong reduction in transmittance in the ultraviolet spectral region, but has little or no effect in the visible and infrared spectral regions. Soft substrates containing halides are unsatisfactory as windows or substrates. Materials choice for dielectric layers should also reflect such considerations. Best performance is also found for the harder materials. Compaction of the layers and interlayer diffusion causes a blue shift in center wavelength and loss of throughput. For sensors using CCD's, shifts in gate voltage and reductions in transfer efficiency occur. Such effects in CCD's are in accord with expectations of the effects of the radiation dose on the device. Except for optical fiber, degradation of CCD's represents the only ionizing-radiation induced effect on the Long Duration Exposure Facility (LDEF) optical systems components that has been observed.

Blue, M. D.↗

DOE OSTI2018

Synthesis and Crystal Structure of the Layered Lanthanide Oxychlorides Ba 3 Ln 2 O 5 Cl 2

Single crystals of a new family of layered lanthanide oxychlorides, Ba 3 Ln 2 O 5 Cl 2 (Ln = Gd–Lu), have been synthesized from a molten barium flux. This family crystallizes in the space group I4/ mmm (No. 139; Z = 2) with lattice parameters a = 4.3384(1)–4.4541(1) Å and c = 24.5108(7)–24.8448(9) Å. Ba 3 Ln 2 O 5 Cl 2 phases are built up of two different blocks: a perovskite double layer of stoichiometry Ba 2 Ln 2 O 5 formed by corner-connected LnO 5 tetragonal bipyramids and a puckered rock-salt-like interlayer of composition BaCl 2 . A complete structural study along with bond-valence-sum calculations shows that, for lanthanides larger than gadolinium, the structure becomes unstable. Density functional theory calculations show that the valence-band edge is dominated by oxygen orbitals, whereas the conduction band forms from Ba 5d orbitals. Finally, the synthesis of this family suggests a route to other potential multianion phases.

36 MATERIALS SCIENCE↗

DOE OSTI2017

Emergent Ising degrees of freedom above a double-stripe magnetic ground state [Emergent Ising degrees of freedom above double-stripe magnetism]

Here, double-stripe magnetism [Q=(π/2,π/2)] has been proposed as the magnetic ground state for both the iron-telluride and BaTi 2 Sb 2 O families of superconductors. Double-stripe order is captured within a J 1 –J 2 –J 3 Heisenberg model in the regime J 3 >> J 2 >> J 1 . Intriguingly, besides breaking spin-rotational symmetry, the ground-state manifold has three additional Ising degrees of freedom associated with bond ordering. Via their coupling to the lattice, they give rise to an orthorhombic distortion and to two nonuniform lattice distortions with wave vector (π,π). Because the ground state is fourfold degenerate, modulo rotations in spin space, only two of these Ising bond order parameters are independent. Here, we introduce an effective field theory to treat all Ising order parameters, as well as magnetic order, and solve it within a large-N limit. All three transitions, corresponding to the condensations of two Ising bond order parameters and one magnetic order parameter are simultaneous and first order in three dimensions, but lower dimensionality, or equivalently weaker interlayer coupling, and weaker magnetoelastic coupling can split the three transitions, and in some cases allows for two separate Ising phase transitions above the magnetic one.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2017

Hydrothermal synthesis and crystal structure of a new lithium copper bismuth oxide, LiCuBiO{sub 4}

A new lithium copper bismuth oxide, LiCuBiO{sub 4} was prepared by hydrothermal reaction using NaBiO{sub 3}0.1*4H{sub 2}O. The crystal structural model of this compound was refined by using synchrotron X-ray powder diffraction data. This bismuthate has the LiCuSbO{sub 4} related structure with the orthorhombic cell (Space group: Pnma) of a=10.9096(9), b=5.8113(5) and c=5.0073(4) Å, and the final R-factors were R{sub wp}=4.84 and R{sub p}=3.58%. This compound is the first example of a lithium copper bismuthate containing Bi{sup 5+}. An antiferromagnetic ordering of Cu{sup 2+} moment was observed at 6 K. - Graphical abstract: In the crystal structure of LiCuBiO{sub 4} all metal atoms are coordinated octahedrally by six O atoms and LiO{sub 6} and CuO{sub 6} octahedra form the one-dimensional chains by edge-sharing along the b-axis. The LiO{sub 6} and CuO{sub 6} chains form the layer by face-sharing in the bc plane. The Bi atoms are placed in that interlayer and BiO{sub 6} octahedra are edge-sharing with LiO{sub 6} and CuO{sub 6} octahedra. - Highlights: • A new lithium copper bismuth oxide, LiCuBiO{sub 4} is prepared by hydrothermal reaction. • The crystal structure of LiCuBiO{sub 4} is closely related with that of LiCuSbO{sub 4}. • This new compound exhibits an antiferromagnetic ordering of Cu{sup 2+} moment at 6 K.

36 MATERIALS SCIENCE↗

DOE OSTI2019

A Natural 2D Heterostructure [Pb 3.1 Sb 0.9 S 4 ][Au x Te 2– x ] with Large Transverse Nonsaturating Negative Magnetoresistance and High Electron Mobility

We report the two-dimensional (2D) natural hetero-structure [Pb 3.1 Sb 0.9 S 4 ][Au$_x$Te$_{2-x}$] ($x$ = 0.52-0.36) which shows anomalous, transverse nonsaturating negative magnetoresistance (MR). For $x$ = 0.52, the material has a commensurately modulated structure with alternating [Pb3.1Sb0.9S4] rocksalt layers and atomically thin [Au$_x$Te$_{2-x}$] sheets, as determined by single-crystal X-ray diffraction using a (3 + 1)-dimensional space group; for other $x$ compositions, the modulated structure is absent and the Au and Te atoms are disordered. The transport properties in this system at low temperature (<100 K) are dominated by an unusual 2D hopping mechanism, while at room temperature a high carrier mobility of similar to ~1352 cm 2 V -1 s -1 obtained ($x$ = 0.36). The confined electrons within the [Au$_x$Te$_{2-x}$] layers are also exposed to interlayer coupling with the insulating [Pb 3.1 Sb 0.9 S 4 ] layers, and as a result, the properties of the heterostructures emerge not only from the constituent layers but also the interactions between them. Furthermore, the various Au and Te coordination patterns found in the [Au$_x$Te$_{2-x}$] sheets as a function of $x$ further contribute to a unique electronic structure that leads to the anomalous nonsaturating negative MR with different field dependent behaviors. First-principles calculations indicate that the [Au$_x$Te$_{2-x}$] sheets are responsible for the unusual electrical transport properties in this 2D system.

2D materials↗

DOE OSTI2021

Anomalous electrodynamics and quantum geometry in the Dirac-Harper model for a graphene bilayer

In this work, graphene bilayers with layer antisymmetric strains are studied using the Dirac-Harper model for a pair of single layer Dirac Hamiltonians coupled by a one-dimensional moiré-periodic interlayer tunneling amplitude. This model hosts low energy, nearly dispersionless bands near charge neutrality that support anomalous polarizations of its charge multipole distributions. These are analyzed introducing a generalized Berry curvature that encodes the field-induced dynamics of multipole fields allowed in a chiral medium with time reversal symmetry. The formulation identifies a reciprocity relation between responses to layer-symmetric and layer-antisymmetric in-plane electric fields and reveals momentum-space quantum oscillations produced by a spatial pattern of band inversions on the moiré scale.

36 MATERIALS SCIENCE↗